By far the most significant process in carrying out a completion in a cased well is that of providing a flow path between the production zone, also known as a formation, and the well bore. Typically, the provision of such a flow path is carried out by using a perforator, initially creating an aperture in the casing and then penetrating into the formation via a cementing layer, this process is commonly referred to as a perforation. Although mechanical perforating devices are known, almost overwhelmingly such perforations are formed using energetic materials, due to their ease and speed of use. Energetic materials can also confer additional benefits in that they may provide stimulation to the well in the sense that the shockwave passing into the formation can enhance the effectiveness of the perforation and produce an increased flow from the formation. Typically, such a perforator will take the form of a shaped charge. In the following, any reference to a perforator, unless otherwise qualified, should be taken to mean a shaped charge perforator.
A shaped charge is an energetic device made up of a housing within which is placed a typically metallic liner. The liner provides one internal surface of a void, the remaining surfaces being provided by the housing. The void is filled with an explosive which, when detonated, causes the liner material to collapse and be ejected from the casing in the form of a high velocity jet of material. This jet impacts upon the well casing creating an aperture, the jet then continues to penetrate into the formation itself, until the kinetic energy of the jet is overcome by the material in the formation. The liner may be hemispherical but in most perforators is generally conical. The liner and energetic material are usually encased in a metallic housing, conventionally the housing will be steel although other alloys may be preferred. In use, as has been mentioned the liner is ejected to form a very high velocity jet which has great penetrative power.
Generally, a large number of perforations are required in a particular region of the casing proximate to the formation. To this end, a so called gun is deployed into the casing by wireline, coiled tubing or indeed any other technique known to those skilled in the art. The gun is effectively a carrier for a plurality of perforators that may be of the same or differing output. The precise type of perforator, their number and the size of the gun are a matter generally decided upon by a completion engineer based on an analysis and/or assessment of the characteristics of the completion. Generally, the aim of the completion engineer is to obtain an appropriate size of aperture in the casing together with the deepest possible penetration into the surrounding formation. It will be appreciated that the nature of a formation may vary both from completion to completion and also within the extent of a particular completion. In many cases fracturing of the perforated substrate is highly desirable.
Typically, the actual selection of the perforator charges, their number and arrangement within a gun and indeed the type of gun is decided upon by the completion engineer. In most cases this decision will be based on a semi-empirical approach born of experience and knowledge of the particular formation in which the completion is taking place. However, to assist the engineer in his selection there have been developed a range of tests and procedures for the characterisation of an individual perforator's performance. These tests and procedures have been developed by the industry via the American Petroleum Institute (API). In this regard, the API standard RP 19B (formerly RP 43 5th Edition) is used widely by the perforator community as indication of perforator performance. Manufacturers of perforators typically utilise this API standard marketing their products. The completion engineer is therefore able to select between products of different manufacturers for a perforator having the performance he believes is required for the particular formation. In making his selection, the engineer can be confident of the type of performance that he might expect from the selected perforator.
Nevertheless, despite the existence of these tests and procedures there is a recognition that completion engineering remains at heart more of an art than a science. It has been recognised by the inventors in respect of the invention set out herein, that the conservative nature of the current approach to completion has failed to bring about the change in the approach to completion engineering required, to enhance and increase production from both straightforward and complex completions.
There are a large number of widely known shaped charge designs, however many of the designs are merely incremental changes to the pressed density of the explosive or the cone angle of the liner. The largest area of development work has mainly concentrated on improving the penetration by the choice of metal liner, its shape, the casing, the type of high explosive and the methods of initiation of the high explosive. The kinetic energy of the jet from a shaped charge is provided exclusively by the detonative pressure of the explosive which forces the collapse of the liner. This in turn leads to the liner material being ejected at a high velocity. Once the jet is in motion there is no further energy available from the system.
In the past depleted uranium (du) shaped charges have been researched but their use is deemed controversial on environmental grounds even within a military context. Du is substantially uranium 238 with only about 0.3% of uranium 235. Apart from the superior penetrative power of du jets when compared with all other liner materials an additional advantage is that the jets may be regarded as being pyrophoric. This may provide some additional jet/target and/or target/behind armour benefits by imparting additional energy and causing additional damage to a target. This additional energy would be extremely useful in the oil and gas industry to fracture the substrates. However the use of a mildly radioactive substance in a commercial application such as an oil and gas perforation would not be considered appropriate.
Therefore it would be desirable to produce a shaped charge liner whose jet can provide additional energy after the detonative event, without the requirement of using a radioactive constituent.